With regard to renewable energy, solar cells are devices that have characteristics that enable them to convert the energy of sunlight into electric energy. The aim of research often is to achieve solar cell designs with the lowest cost per watt generated by the solar cell, and, concurrently, the designs should provide solar cells that are suitable for inexpensive commercial production.
A conventional thin film solar cell is composed of a stacking of thin layers on a rigid or flexible substrate. The thin layers form one or more junctions that absorb light and convert it into electricity. Briefly, a typical thin film photovoltaic (PV) device, such as a thin film solar cell, may include a glass, metal, or polymer substrate, a back contact, an absorber, a window layer, a front contact or low resistivity layer, and a top protective layer (e.g., a glass substrate) or a similar arrangement of thin film layers. An alternative arrangement would be a transparent superstate (e.g., glass or polymer), a front contact layer, a buffer layer, a window layer, an absorber, a back contact, and a protective backsheet.
Presently, most thin film solar cells are fabricated with an absorber or absorber layer formed of cadmium telluride (CdTe) or copper indium gallium selenide (CIGS). An absorber formed of either material has a high optical absorption coefficient and suitable optical and electrical characteristics. With regard to CdTe solar cells, much of the recent research efforts has been directed at producing CdTe structures that allow more light to penetrate the top layers of the device (e.g., the transparent conducting contacts, the buffer layer, and the window layer, which is often formed of cadmium sulfide (CdS)) to achieve high efficiency. While with CIGS solar cells, work continues to provide better methods of producing a CIGS thin film layer that is of proper composition and structure to allow charges generated by received sunlight (i.e., electrons and holes) to exist long enough in the CIGS layer of the device so that they can be separated and collected at the front and back contacts to provide higher conversion efficiency.
A modification to solar cells was made when the current, voltage, and fill factor of the cells were found to be limited by a number of factors including roughness of the superstate and front contact layer, pinholes in the window and absorber layer, space charge collapse, and other factors. These factors were all exacerbated when cells were scaled up in size to make larger area products called modules. Solar cell modules were found to be more sensitive to these effects than small solar cells, and it was found useful to provide an additional layer called a buffer layer (as well as an intrinsic layer or a high-resistive transparent (HRT) layer) between the front contact layer and the window layer. To allow light to pass to the absorber layer, the buffer layer typically is formed of a transparent, moderately conducting oxide such as tin oxide (SnO2) and serves many functions in the cell or module while having a high enough resistivity to both match the window (e.g., CdS) layer and provide adequate protection against shunting from the transparent conductive oxide (TCO) to the absorber when the window layer is relatively thin and/or contains pinholes.
While sometimes labeled a “high resistivity” layer, it is more accurate to think of this layer as a minimally conductive layer because the cell's function is enhanced when the buffer layer has a more intermediate resistivity. However, the PV and glass industries have found it difficult to manufacture a buffer layer with intermediate resistivity especially when employing commercial deposition processes and industry standard precursors (e.g., chlorine (Cl)-containing organo-tin compounds). In contrast, it is often straightforward to produce a highly resistive buffer layer or a highly conductive buffer layer. There remains a need for a method of providing a buffer layer that is minimally conductive while still providing other desired properties for a thin film PV device including near zero optical absorption and low surface roughness. It is also desirable that the resistivity (and conductivity) of the buffer layer be readily tunable to suit the design of the absorber and other layers of the PV stack.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.